This invention relates to a magnetic bubble memory device, and more particularly to improvements in a magnetic bubble generator in such memory device.
The magnetic bubble memory device is well known to those skilled in the art. One mode of operation of such storage device is called the "field access". The reason is that the movement of magnetic bubbles within a thin film of a magnetic bubble material is conducted in response to a magnetic field rotating within the plane of the thin film.
In a typical case, the thin film is an epitaxial film of garnet. The magnetic bubbles move within the thin film along a propagation path which is defined by the cyclic patterns of a soft magnetic material (high permeability), typically permalloy. In response to the magnetic field rotating within the plane of the thin film, the material generates magnetic poles, which give rise to a localized field gradient for moving the magnetic bubbles within the thin film.
The magnetic bubble memory device of this type usually has the "major/minor loop" or "major line/minor loop" organization. In either memory organization, the magnetic bubble generator is disposed in the major loop or the major line. The magnetic bubble generators are broadly classified into the seed bubble type and the nucleation type. Presently conventional among them is the magnetic bubble generator of the latter or the nucleation type. In this type, a hairpin-shaped conductor loop is disposed between the magnetic bubble propagation path and the thin film of the magnetic bubble material in a manner to be insulated therefrom, and a pulse current is caused to flow through the loop, whereby a magnetic bubble is generated in the part of the thin film corresponding to a stable point on the magnetic bubble propagation path. FIG. 1 shows the construction of a prior-art, nucleation type magnetic bubble generator. Referring to the figure, numeral 1 designates a propagation path, and numeral 2 a conductor loop. The propagation path 1 is exemplified as employing asymmetric chevron elements 3. Numeral 4 designates an element which has a shape suitable for generating a magnetic bubble 9 and which is called the "pickax element". The pickax element 4 is also one element which constitutes the propagation path 1 of the magnetic bubble 9. The magnetic bubble generator is constructed of the conductor loop 2 and the pickax element 4. Here, a rotating field H.sub.R is rotating counterclockwise as indicated in the figure. In response to this rotating field H.sub.R, accordingly, the magnetic bubble 9 is propagated leftwards (in the direction of arrow 5) along the propagation path 1. For the sake of convenience, a vertically downward position shall be selected 0.degree. as the reference of the direction of the rotating field H.sub.R as indicated in the figure. Further, for the sake of convenience, a bias field H.sub.B is supposed to be acting in a direction out of the paper as indicated in the figure. In this case, when the direction (phase) of the rotating field H.sub.R is 180.degree., the end face A of the pickax element 4 surrounded with the conductor loop 2 becomes the stable existence position of the magnetic bubble 9.
The generation of the magnetic bubble 9 is effected by causing a pulse current I.sub.G to flow into the conductor loop 2 in the direction of arrow 6 when the direction of the rotating field H.sub.R is near 180.degree.. When the pulse current I.sub.G flows through the conductor loop 2, a magnetic field established in a region B surrounded with the conductor loop 2 is in a direction into the paper. That is, this magnetic field acts so as to weaken the bias field H.sub.B in the region B. Owing to the pulse current I.sub.G, the magnetic bubble 9 is generated at the end face A of the pickax element 4. The magnetic bubble 9 thus generated is propagated from the element 4 to the element 3 or leftwards along the propagation path 1 in response to the rotation of the rotating field H.sub.R. Depending on whether or not the pulse current I.sub.G is caused to flow into the conductor loop 2, the magnetic bubble 9 is existent or nonexistent in the element 3 constituting the propagation path 1. By bringing the "presence" and "absence" of the magnetic bubble 9 into correspondence with "1" and "0" of information denoted by the binary system, respectively, a train of information can be formed on the propagation path 1.
Such prior-art magnetic bubble generator has a problem as stated below.
The problem is that when the operation of generating the magnetic bubble 9 is repeated over a long time, a malfunction can occur. In the malfunction, an undesirable extra magnetic bubble 10 is generated in the region B shown in FIG. 1. The magnetic bubble 10 moves round the magnetic bubble generator, finally enters the propagation path 1, and forms a cause for disordering the train of information. In a graph of FIG. 2, the ordinate represents pulse current, the abscissa represents the surface temperature of the chip of a magnetic bubble memory, and the upper limit values (straight line 7) and lower limit values (straight line 8) of the amplitude of the pulse current I.sub.G allowable at respective temperatures as have been obtained are connected as the straight lines. The lower limit value of the amplitude of the pulse current I.sub.G is a current value which is required for generating a magnetic field sufficient to weaken magnetization induced by the bias field H.sub.B in the direction thereof and to turn it into the opposite direction. On the other hand, the upper limit value of the amplitude of the pulse current I.sub.G is not considered to exist specifically, but it is actually existent as stated above. More specifically, the upper limit value is determined by that amplitude of the pulse current I.sub.G at which the extra magnetic bubble 10 described above is generated in a fixed proportion. The straight line 7 in FIG. 2 indicates the upper limit values which correspond to the amplitudes of the pulse currents I.sub.G in the case where one extra magnetic bubble 10 is generated in the region B when 10.sup.8 magnetic bubbles 9 have been generated by the generator.
As apparent from the tendency of the straight line 8 in FIG. 2, the lower limit value is not problematic because it lowers only slightly even when the temperature of the chip has risen. In contrast, the upper limit value lowers greatly with the rise of the temperature of the chip as apparent from the tendency of the straight line 7. This signifies that when the temperature of the chip has become high, the width between the upper and lower limits of the allowable amplitude of the pulse current I.sub.G, i.e., the current margin becomes very small. Therefore, any countermeasure has been eagerly desired.